A Survey on Cooperative Diversity for Wireless Networks F

IEEE COMMUNICATIONS SURVEYS & TUTORIALS, ACCEPTED FOR PUBLICATION 1 A Survey on Cooperative Diversity for Wireless Networks F. Gómez-Cuba, R. Asorey-Cacheda and F.J. González-Castaño Abstract—Diversity, i.e. transmitting multiple replicas of a signal, may mitigate fading in wireless networks. Among other diversity techniques, the space diversity of multi-antenna systems is particularly interesting since it can complement other forms of diversity. The recent cooperative diversity paradigm brings the advantages of multi-antenna space diversity to single antenna networked devices, which, through cooperation and antenna sharing, form virtual antenna arrays. However, cooperative H diversity is a complex technique and research on this topic is still in its early stages. This paper aims at providing a general survey on the theoretical framework; and the physical and medium access control proposals in the literature. Index Terms—Wireless networks, cooperative diversity, MAC protocols. S D I. INTRODUCTION Due to time-variant fading, the attenuation in a wireless channel may vary due to multiple circumstances. Thus, wireless system designs typically include some degree of diversity so as to provide the receiver with several realizations of the signal, which increases the chances of a successful transmission [1]. Fig. 1. With cooperation, more nodes are subject to interference. In this case, Many forms of diversity are possible depending on how the helper (H) that receives the transmission of the source (S) and forwards it to the destination (D), generates extra interference that would not be present different available channels or subchannels replicate the signal. if direct transmission was used. Time diversity consists of transmitting replicas with enough separation in time to allow signal decorrelation. Frequency diversity relies on multiple carriers, and space diversity sys- better reception in each receiver along the path and, thus, tems have multiple antennae that are sufficiently spaced and improvements in range, rate or autonomy. These tradeoffs are receive the same information [1]. analyzed in [1] and [3], which conclude that the strategy is Wireless user devices tend to be constrained in size, com- profitable. For this reason, future wireless network designs plexity and power, rendering previous diversity methods un- should consider cooperation capabilities. feasible. To cope with this problem, the cooperative diversity Cooperative diversity, as a technique to combat fading, paradigm has appeared recently. In it, single-antenna net- should find its niche in the upcoming generations of mo- worked nodes coordinate themselves to form a virtual antenna bile data networks, typically cellular architectures, granting array, seeking the advantages of MIMO spatial diversity [1]: a higher throughput by means of spectrum reutilization. Con- each source associates itself with other nodes, acting as helpers sequently, the cooperative scenario requires new analyses as that first receive the transmission of the source and then the introduction of third parties tends to increase interference relay the information. As a result, one extra transmission is [4], [5] (see Figure 1). needed to send the information to the receiver and the number Although mobile cellular networks are the natural target of hops in each route is doubled. The increase in cost is of cooperative diversity techniques, any wireless network only due to the second stage, since the broadcast nature of affected by fading can use them. Since the benefit increases the wireless network allows simultaneous transmission to as as more potential helpers conform the network, dense sensor many helpers as needed in the first stage. Furthermore, multi- networks [6], [7] represent a good application scenario for hop transmission [2] with cooperative diversity may favor a low complexity cooperative techniques. A cooperative virtual backbone may also be of interest in ad-hoc networks [8], [2]. Manuscript received 22 March 2011; revised 26 July 2011. F. Gómez-Cuba is with Gradiant, CITEXVI, Campus, 36310 Vigo, Spain. Cooperation is a versatile strategy, which can be exploited R. Asorey-Cacheda and F.J. González-Castaño are with the Departamento de for purposes other than diversity. Cetinkaya and Orsun pro- Enxeñaría Telemática, Universidade de Vigo, ETSI Telecomunicación, Cam- posed a MAC protocol in which nodes cooperate to adapt their pus, 36310 Vigo, Spain (e-mail: [email protected]; [email protected]; [email protected]). contention windows, improving fairness [6]. Nevertheless, Digital Object Identifier 10.1109/SURV.2011.082611.00047 cooperative diversity differs from other cooperation techniques 1553-877X/11/$25.00 c 2011 IEEE 2 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, ACCEPTED FOR PUBLICATION Phase-I S tx. S tx. S Phase-II D 1 2 2 2 T T 2 2 Fig. 3. TDMA medium sharing without cooperation. The assignment period T is divided in two medium accesses. S1 tx. S2 relays S1 S2 tx. S1 relays S2 T T T T S D 4 4 4 4 1 1 Fig. 4. TDMA medium sharing with cooperation.The assignment period T is divided in four slots, encompassing two direct transmissions and two relay transmissions. The same information is transmitted twice and the rate is halved. Fig. 2. Wireless network example: Two sources S1 and S2 access the medium alternately to transmit to the two destinations D1 and D2; both nodes of each communication pair overhear the transmission of the other It is possible -by halving the transmission rate- to allow pair. a node to allocate half its transmission time to its own information and the other half to relaying information (figure in the sense that the improvement opportunities rely on the 4). physical layer (PHY). Consequently a cross-layer design is Using the above forwarding policy, the overall outage O(P 2) required if other layers are involved. probability would be of the order of o , because a packet The rest of this paper is structured in four parts. Section II would only get lost in the case of outage in two independent reviews the background from the perspective of information routes (one composed of two links, with the helper in be- theory, emphasizing the fact that a point-to-point approach tween). Evidently, this model is oversimplified: link outages cannot achieve the full capacity of a network [9]. Different are assumed to be independent, the PHY layer processes theoretical transmission models are reviewed and compared. individual packets independently, the helper must fully (and Section III analyzes recent PHY architectures. Relying on successfully) decode the packets it retransmits, the destination information theory, we discuss a representative group of decodes each replica independently, and at least one of the cooperative PHY techniques. Section IV surveys cooperative replicas must be successfully decoded at the destination. However, if we analyze the problem from the perspective MAC layers. Finally, section V concludes the paper. of information theory as in [10], the system entangles much more mutual information than that defined by the hop-by- II. INFORMATION THEORETICAL APPROACH hop success-requiring characterization. Mutual information is A. Philosophy given by the combination of the direct link and the two-hop Let us consider a four-node wireless network with two link from a source to a destination. This means that the desti- transmitters S1 and S2 and two destinations D1 and D2 as nation can use information from both the direct transmission shown in figure 2. The two transmitters share the medium and the relayed transmission in order to decode the data. by some means, such as, for instance, pre-assigned periodic Laneman et al [11] considered two classical relay algorithms symmetric TDMA slots (figure 3). Assuming a Rayleigh depending on whether or not the relay/helper decoded the fading coefficient α that is slowly varying and flat, the link received signal, (decode-and-forward (DF) and amplify-and- outage probability P0 is defined as the probability that the forward (AF), respectively). In cooperative AF, an optimum signal-to-noise ratio seen by the receiver (|α|2SNR,where receiver accesses the information of two parallel noisy chan- SNR is the signal-to-noise ratio at the transmitter) is lower nels, one of which has a classical AF relay. Regarding DF, than the minimum needed, i.e. the rate is above the mutual either the source-to-relay or the relay/source-to-destination information of the channel [10]: links limit the maximum achievable rates. R − P P R> |α|2SNR P |α|2 < 2 1 o = log2(1 + ) = SNR B. Theoretical System Models (1) Based on classical relay techniques, Laneman et al suggest where R is the binary rate per Hertz, defined per slot. some improvements [10]: It is remarkable that, due to the broadcast nature of the • Selection Relaying uses either AF or DF and, if the relay wireless medium, a node that is idle at a certain moment reception signal to noise ratio (SNR) is low, the relay can overhear the transmissions of its peers. In typical wireless stays silent, allowing the source to retransmit instead. systems these receptions are simply discarded. However, since • Incremental Relaying isbasedontheassumptionofsome different nodes experience independent realizations of the kind of acknowledgement. So, regular communication fading phenomena, the success probabilities are independent takes place first and, if no acknowledgement (ACK) is and, thus, a transmitter can rely on a helper to create a form received, the relay sends its copy of the signal to the of diversity. destination. GOMEZ-CUBA et al.: A SURVEY ON COOPERATIVE DIVERSITY FOR WIRELESS NETWORKS 3 d(R ) norm Ch1 S1 tx. S2 relays S2 S3 relays S2 SN relays S2 Ch S tx. S relays S S relays S S relays S 2 DF 2 2 1 2 3 2 N 2 Ch S tx. S relays S S relays S S relays S Direct 3 3 1 3 2 3 N 3 AF Incremental AF ChN SN tx.

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